Adherent coatings by immersion plating from non-aqueous solutions

ABSTRACT

A METHOD OF APPLYING ADHERENT PLATED COATINGS ON ARTICLES CONTAINING AT LEAST ONE OF THE METALS SELECTED FROM THE GROUP CONSISTING OF ALUMINUM, BERYLLIUM, TITANIUM, NIOBIUM AND ZIRCONIUM INCLUDES THE STEPS OF (1) IMMERSING THE ARTICLE IN A METHANOL SOLUTION OF A HALIDE OF THE PLATING METAL, AND (2) REMOVING THE ARTICLE FROM THE SOLUTION WHEN THE DESIRED PLATED COATING IS OBTAINED.

United States Patent ADHERENT COATINGS BY IMMERSION PLATING I FROM NON -AQUEOUS SOLUTIONS David S. Kneppel, Needham, Mass., assignor to Whittaker Corporation, Nuclear Metals Division, West Concord, Mass. No Drawing. Filed Apr. 1, 1968, Ser. No. 717,950 Int. Cl. C23c 3/00 US. Cl. 117-130 5 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION (a) Field of the invention The present invention relates to plated metal coatings. More particularly, it relates to a method of applying adherent plated coatings on certain metals which are difficult to plate.

(b) Description of the prior art The three types of plating widely used at present are electroplating, electroless plating and immersion plating. Electroplating, as is well known, involves'the use of a DC. circuit consisting of a source of direct current and an anode and cathode disposed in an electrolytic bath. With this arrangement, the plating metal is used as an anode which is depleted as ions of the plating metal migrate to the cathode where they are reduced to form a coating on the base metal comprising the cathode.

In electroless plating no electric current is used and the base metal is plated by reduction of the plating metal from a solution of a salt of the plating metal. This solution must contain reducing agents to accelerate chemical reduction of the plating metal onto the base metal.

Immersion plating, like electroless plating, does not employ an electric current. Although immersion plating also utilizes a solution of a salt of the plating metal in which the base metal is immersed, in immersion plating the plating metal deposits on the base metal by means of displacement rather than reduction. In other words, the plating metal must be below hydrogen in the electromotive seriesand the base metal above hydrogen" in order for the substitution of the base metal for the plating metal in solution to occur. p

While electroplating is very widely used to provide adherent plated coatings of almost any desired thickness, it is not suitable for use with tiny articles, such as shot, fibers, and granular or other particulate material. This is true principally because it is difficult to utilize such articles or materials as a cathode in'a platingfbath'For this reason, either electroless or immersion plating is preferred where uniform plated coatings aredesired on very tiny articles'or particulate material. j

Although'the physical characteristics of the material to be plated are of significance in determining the proper selection of plating technique, the chemical constituency of the base material is even more important. For 'example, some base metals are exceedingly difficult to plate with adherent coatings by any of the three'principal plating techniques. The so-called valve metals, such as aluminum, tantalum, titanium and niobium readily form 3,558,349 Patented Jan. 26, 1971 adherent oxide coatings which are electrically resistant. Indeed, this is why the valve metals are anodized and used as electrodes in electrolytic capacitors. Because of this tendency to form oxide coatings in water or water solutions, the valve metals are exceedingly diflicult to plate. Generally if any oxide is present on the surface of these metals at the time they are plated, the plated coating will not be adherent. It has therefore been necessary to devise plating techniques which do not employ water solutions. One such approach is illustrated in US. Pat. No. 3,28l,264Cape et al. which teaches a method of gold plating aluminum. This involves brushing on a solution of gold chloride in an essential oil carrier. The coating is then heated to a temperature of 1600 F. to 1800 F. thereby reducing the gold chloride and volatilizing away the carrier.

Another approach involves immersion coating from a non-aqueous solution of a plating metal more noble than the base metal. This process, disclosed in US. Pat. No. 2,873,214-Schnable, also requires the use of high temperatures since the bath is maintained at a temperature below the solidus point of the base metal but above the liquidus point of the metal coating.

Yet another elfort employing the use of non-aqueous plating techniques is illustrated in US. Pat. No. 3,265,526 to Beer. Here, plating with precious metals is accomplished using salts of the metals in organic solvents together with a reducing agent. The metal and coating are then heated to effect an adherent plating. The techniques taught in each of the three foregoing patents, therefore,

suitable for the plating of metals such as beryllium or titanium which readily form oxide coatings that impair the tenacity of a plated coating.

It is, therefore, an object of the present invention to provide an inexpensive, low temperature method of applying adherent coatings on diflicult-to-plate metal substrates such as beryllium, titanium, aluminum, niobium and zirconium.

Other objects of the invention will in part be obvious .and will in part appear hereinafter.

SUMMARY OF THE INVENTION Briefly, my method includes the steps of (1) immersing an article containing one or more of the foregoing metals in a room temperature methanol solution of a halide of the plating metal and (2) removing the article from the solution when the desired plated coating is obtained. This method may be carried on at room temperature, or almost any suitable temperature below the boiling point of the solution, thereby eliminating the need for an expense of carrying on a part of the plating process at high temperatures. It also provides a strongly adherent plated coating on base metals such as beryllium, aluminum, titanium, niobium, zirconium, and alloys and solutions of these metals. Even in cases where electroplating is preferred, my methanol solution may be used with a plating current to provide results not readily attainable with the typical plating solutions using water as the solute.

DESCRIPTION OF THE PREFERRED EMBODIMENT As pointed out above, I immersed articles made from one or more of the foregoing base metals in an anhydrous methyl alcohol solution of the halide of a desired plating metal. Preferably, the halide is the chloride of a metal below hydrogen in the electromotive series. The following examples illustrate my method in more detail.

EXAMPLE I A piece of titanium was immersed in a solution comprising about 2 gms. of anhydrous copper chloride (CuCI dissolved in 100 cc. of reagent grade absolute methyl alcohol. After a few minutes the titanium was removed from the solution and the surface rinsed under water and wiped dry. A-n adherent copper coating on the titanium was obtained. It should be noted, however, that in this and the following examples, trace amounts of water are actually present in the CuCl and that the presence of minute amounts of water are actually beneficial to the process.

Removal of the oxide scale on the base metal prior to plating by my method is not essential for obtaining an adherent deposit. On the other hand, however, an oxide free surface, such as might be obtained by mechanical abrasion, is desirable for more rapid and uniform plating. Chemical removal of the oxide scale is also suitable as a substitute for mechanical abrasion.

EXAMPLE II A beryllium strip was immersed in 100 ml. of anhydrous methanol containing .3 gm. of anhydrous CuCl An adherent copper coating on the beryllium was obtained after several minutes immersion in the solution.

This experiment was repeated several times using increased concentrations of CuCl between 0.3 gm. per 100 ml. up to 6 gms. CuCl per 100 ml. of anhydrous methanol. It was noted that the deposition rate of copper on the beryllium varies with the concentration.

and appears to be directly proportional to the concentration.

EXAMPLE III Several strips of aluminum were immersed in 100 ml. of anhydrous methanol containing 3 gms. of CuCl One strip was immersed for 3 minutes, one for 6 minutes and one for 30 minutes.

Upon removal, the copper thickness on the aluminum was determined by metallographic measurements. That is, the samples were cross sectioned, the edges were polished and the thickness of the plated coating was metallographically measured. The sample immersed for 3 minutes had a coating thickness of approximately .1 to .2 mil. The sample immersed for 6 minutes had a coating thickness of .3 to .4 mil, and the sample immersed for 30 minutes had a coating thickness of approximately .5 mil.

Since no reducing agent is specifically added to the plating solution used in the method of my invention, it would appear that my method is one of immersion plating rather than electroless plating. On the other hand, however, the results of the above example tend to belie this fact since the coating thickness increases, though not linearly, with time. This phenomenon is contrary to what one would expect as a result of a displacement reaction since plating by chemical displacement tends to be self-limiting. In other words, once the surface molecules or atoms on the base metal are displaced and no further base metal atoms remain exposed, no further displacement can occur. One explanation for the phenomenon which occurs in the practice of my method, however, is that it is one of electroless plating and the methanol is acting as a reducing agent.

Other organic solvents such as ethanol have been tried and found not to be satisfactory. For example, ethanol has been found to be completely unsatisfactory as a halide solvent for use in plating beryllium or titanium. It has also been found unsatisfactory as a solvent for 4 plating Lockalloy (Trademark of the Lockheed Aircraft Corporation for a beryllium aluminum alloy).

In various other experiments salts other than halides, as for example nitrates and sulfates of the plating metals, have been tried and under experimental conditions, like those used in the above examples, have also been found to be unsatisfactory.

While my method can also be used to deposit some metals above hydrogen in the electromotive series, it has not been found to be especially eflicient for use with such metals. For example, an anhydrous methanol solution of nickel chloride may be used to apply an adherent nickel coating on any one of the aforesaid base metals. The plating time, however, is significantly reduced when a current is used. In such a case, the methanol solution of nickel chloride is used as the electrolyte in a conventional plating bath. The base metal then serves as a cathode and nickel is the anode. Also, my method can be used to provide a base coating, such as copper, for subsequent electroplating of metals, such as chromium, which do not readily plate on a variety of substrates.

The results obtained with my method confirm that it is possible to provide adherent deposits on a variety of base metals such as titanium, zirconium, beryllium, niobium and aluminum. Such base metals can be uniformly coated regardless of their size or configuration, even if they be in particulate or granular form. No special treatment of the base metal is required prior to the plating process and the plating process may be carried on at room temperature.

It will thus be seen that the objects set forth above, among those made apparent from the preceding description, are efficiently attained and, since certain changes may be made in carrying out the above method without departing from the scope of the invention, it is intended that all matter contained in the above description shall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the following claims are intended to cover all of the generic and specific features of the invention herein described, and all statements of the scope of the invention which, as a matter of language, might be said to fall therebetween.

I claim:

1. A method of electrolessly immersion coating adherent layers of a metal below hydrogen in the electromotive series onto articles comprising at least one of the metals selected from the group consisting of aluminum, beryllium, titanium, niobium and zirconium, said method comprising:

(A) immersing the article in a substantially water free methanol solution of a halide of the plating metal, said solution being at a temperature less than its boiling point; and

(B) removing the article from said solution when the desired plated coating is obtained.

2. The method of claim 1 wherein said halide is a chloride.

3. The method of claim 1 wherein the concentration of the salt in solution varies between 0.1 gm. per ml. of absolute methyl alcohol and saturation.

4. The method of claim 3 wherein the concentration of salt is between 0.1 and 6 gms. per 100 ml. of methanol.

5. The method of claim 4 wherein the salt is CuCl- References Cited UNITED STATES PATENTS RALPH L. KENDALL, Primary Examiner US. Cl. X.R. 

